The present invention relates generally to the field of traveling wave devices or tubes, and more specifically to the production of such devices having a greatly increased RF output, reduction in weight and substantially reduced production costs through design simplification and new production techniques.
During the past two decades traveling wave tubes have become the most prominent electron device for the generation and amplification of RF power in the microwave and millimeter bands of the transmission spectrum. Two typical uses of traveling wave tubes are in the communication and the guidance fields. There are other well known fields for these devices.
The traveling wave tube derives its name from the manner in which DC energy is converted to RF energy. This conversion is accomplished by a traveling but delayed electromagnetic wave interacting with an electron beam in the same direction and at substantially the same velocity or "travel". It is essential to the successful operation of a traveling wave tube that there be a transfer of DC beam energy to the electromagnetic circuit wave at synchronism of the beam and the wave velocity. This traveling and distributed interaction explains the dominance at the higher frequencies of the traveling wave devices over grid controlled tubes in which the interaction fields are stationary and with shorter wave lengths which limit the interaction energy transfer. The key element in a traveling wave tube, which permits the high energy conversion, is a delay line which functions to reduce the velocity of the electromagnetic wave from its natural speed of light to a speed which substantially matches the velocity of the electron beam passing through and coaxial with the delay line. There are two common types of traveling wave devices. The first is known as the O type in which the gainful energy exchange derives from the kinetic energy of the electron beam. The second is known as the M type in which the interaction energy derives from the potential energy of the electron beam.
Of the slow wave structures devised for use in traveling wave devices, the helical configuration is the more common. Both unifilar and multifilar helices have been used. In the O type device a circular unifilar helix with predetermined pitch is the nearly exclusive form used in low and medium output devices, and the operational bandwidths are the widest obtained to date in delay lines. However, such helices (both unifilar and multifilar) when operation is attempted above a definite voltage limit, in an attempt to increase beam power, have a notorious tendency to support backward wave interaction. Such tendency is sufficient for self maintained backward wave oscillations. This is a desireable characteristic when the traveling wave device is used as an oscillator; however, in amplifier applications this characteristic is very detrimental. Thus for a long time, the cross-wound helix called a ring bar circuit, has been used because this circuit does not support the backward mode while at the same time allows for substantially larger peak power output. There is a penalty when using a ring bar circuit in that there is a much smaller bandwidth performance, in addition to higher manufacturing costs. Their principal use is thus in applications where bandwidth limitations are not of prime consideration.
The major technical problem which must be overcome before the power output of traveling wave devices can be increased by any substantial amount, is to find a way for removing and dissipating heat generated in the helical slow wave structures. Since the slow wave structure is coaxial within the body of the traveling wave tube, it must be supported within the tube by means which are dielectric while at the same time forming the only means for the transfer of heat from the slow wave structure to the outer body shell structure which acts as a heat sink.
One method has been to support the slow wave structure by means of a plurality of ceramic rods as illustrated in FIG. 3. It is obvious that this configuration provides the poorest possible heat transfer areas and that heat will be extracted from a relatively small area of the slow wave structure.
The best known published art relating to the present invention is covered by U.S. Pat. No. 3,670,196 Helix Delay Line For Traveling Wave Devices, Burton H. Smith. This structure presents very severe manufacturing problems and would also be costly out of all proportion. There is no knowledge of production traveling wave devices having been built in accordance with this patent.
Traveling wave devices are being built at the present time in which a plurality of diamonds support the slow wave structure within the body of the devices. The diamonds have been found to be a good heat transfer media; however, it is obvious that those areas of the slow wave structure not in direct contact with the diamonds will become "hot spots".